1) Field of the Invention
The present invention relates to an optical receiving apparatus and a method for controlling the optical receiving apparatus suitable for use in an optical transmission system, particularly, an optical transmission system adopting optical phase modulation and demodulation.
2) Background of the Invention
In these years, there is a growing demand for introduction of a 40 Gbit/s optical transmission system in the next generation. In addition, a transmission distance and frequency utilization efficiency equivalent to those of the 10 Gbit/s system are also demanded. RZ-DPSK (Differential Phase Shift Keying) or CSRZ-DPSK, which has excellent optical signal-to-noise ratio (OSNR) tolerance and nonlinearity tolerance as compared with NRZ (Non Return to Zero) employed in known systems of not larger than 10 Gb/s, is being vitally researched and developed as a means meeting the above demands. Besides the above modulation systems, study and development of a phase modulation system such as RZ-DQPSK or CSRZ-DQPSK (Differential Quadrature Phase-Shift Keying) characterized by narrow spectrum (high frequency use efficiency) become vital.
FIG. 30 is a diagram showing an optical transmitting apparatus 110 which adopts RZ-DPSK or CSRZ-DPSK at 43 Gb/s to transmit an optical signal, and an optical receiving apparatus which performs a receiving process such as demodulation and the like on the optical signal modulated in RZ-DPSK or CSRZ-DPSK. When an optical signal is transmitted using RZ-DPSK or CSRZ-DPSK modulation/demodulation system, the optical signal is in a 43 GHz clock waveform as the optical intensity, and information is modulated on binary optical phase.
The optical transmitting apparatus 110 shown in FIG. 30 comprises a transmission data processing unit 111, a CW (Continuous Wave) light source 112, a phase modulator 113 and an RZ pulse-curving intensity modulator 114. The transmission data processing unit 111 has a function as a framer for framing inputted data, a function as an FEC (Forward Error Correction) encoder for giving an error correction code, and a function as a DPSK pre-coder for performing a coding process reflected difference information between a code of the preceding bit and the present code.
The phase modulator 113 modulates a continuous beam from the CW light source 112 with encoded data from the transmission data processing unit 111, and outputs an optical signal in which information is modulated on binary optical phase but whose optical intensity is constant, that is, an optical signal modulated in DPSK, as denoted at A1 and A2 in FIG. 30. The RZ pulse-curving intensity modulator 114 transforms the optical signal from the phase modulator 113 into an RZ pulse. Particularly, an optical signal transformed into an RZ pulse by use of a clock driving signal which is at the same frequency (43 GHz) as the bit rate and has an amplitude one times as large as the extinction voltage (Vπ) is called an RZ-DPSK signal, whereas an optical signal transformed into an RZ pulse by use of a clock driving signal which is at a frequency (21.5 GHz) one half as large as the bit rate and has an amplitude two times as large as the extinction voltage (Vπ) is called a CSRZ-DPSK signal.
The optical receiving apparatus 120 is connected to the optical transmitting apparatus 110 over a transmission path 101 to receive a (CS)RZ-DPSK signal, and performs the receiving process on the signal. The optical receiving apparatus 120 comprises a delay interferometer 121, a photoelectric converting circuit 122, a regenerating circuit 123 and a received data processing unit 124.
The delay interferometer 121 is composed of, for example, a Mach-Zehnder interferometer. The delay interferometer 121 makes one component delayed by one bit time (23.3 ps in this case) and the other component whose phase is controlled to be at 0 radian of the (CS)RZ-DPSK signal transmitted over the transmission path 101 interfere with each other, and provides two outputs as a result of the interference. Namely, one branching waveguide of the Mach-Zehnder Interferometer is formed to have a length longer than the other branching waveguide by a propagation length corresponding to one bit time, and the delay interferometer 121 is provided with an electrode 121a for controlling the phase of the optical signal propagated in the latter branching waveguide.
The photoelectric converting unit 122 is composed of a dual pin photodiode which receives the two outputs from the above delay interferometer 121 to perform balanced detection. The received signal detected by the above photoelectric converting unit 122 is appropriately amplified by an amplifier 122c. The regenerating circuit 123 extracts a data signal and a clock signal from the received signal balanced-detected by the photoelectric converting unit 122. The data processing unit 124 performs signal processing such as error correction and the like on the basis of the data signal and the clock signal extracted by the regenerating circuit 123.
FIG. 31 is a diagram showing an optical transmitting apparatus 130 adopting 43 Gb/s RZ-DQPSK or CSRZ-DQPSK to transmit an optical signal, and an optical receiving apparatus 140 performing the receiving process on the optical signal modulated in RZ-DQPSK or CSRZ-DQPSK. When the optical signal is transmitted and received in RZ-DQPSK or CSRZ-DQPSK modulation/demodulation system, the optical signal has a 21.5 GHz clock waveform as the optical intensity, and information is modulated on quaternary optical phase. Hereinafter, the structure for transmitting and receiving data in the above-mentioned RZ-DQPSK or CSRZ-DQPSK modulation/demodulation system will be schematically described, the details of which are described in Published Japanese Translation of PCT International Publication for Patent Application No. 2004-516743, for example.
The optical transmitting apparatus 130 shown in FIG. 31 comprises a transmission data processing unit 131, a 1:2 demultiplexing (DEMUX) unit 132, a CW (Continuous Wave) light source 133, a π/2 phase shifter 134, two phase modulators 135-1 and 135-2, and an RZ pulse-curving intensity modulator 136.
The transmission data processing unit 131 has functions as a framer and an EFC encoder similar to those of the transmission data processing unit 111 shown in FIG. 30, and a function as a DQPSK precoder for performing an encoding process reflected difference information between a code of the preceding bit and the present code. The 1:2 demultiplexing unit 132 demultiplexes a 43 Gbit/s encoded data from the transmission data processing unit 131 into two sequences of the 21.5 Gbit/s encoded data (data #1 and data #2).
The CW light source 133 outputs a continuous beam. The continuous beam outputted from the CW light source 133 is branched into two, and one beam branched is inputted to phase modulator 135-1, whereas the other branched beam is inputted to the phase modulator 135-2 via the π/2 phase shifter 134. The phase modulator 135-1 modulates the continuous beam from the CW light source 133 with the encoded data (data #1) in one sequence demultiplexed by the 1:2 demultiplexing unit 132, and outputs an optical signal in which information is modulated on binary optical phase (at 0 radian or π radian).
The phase modulator 135-2 is inputted a continuous beam obtained by shifting the phase of the continuous beam from the CW light source by only π/2 by means of the π/2 phase shifter 134, modulates the inputted continuous beam with encoded data (data #2) in the other sequence demultiplexed by the 1:2 demultiplexing unit 132, and outputs an optical signal in which information is modulated on binary optical phase (at π/2 radian or 3π/2 radian).
The modulated beams from the above phase modulators 135-1 and 135-2 are combined, and outputted to the RZ pulse-curving intensity modulator 136 in the following stage. Namely, the modulated beams from the phase modulators 135-1 and 135-2 are combined, whereby an optical signal, in which the optical intensity is constant but information is modulated on quaternary optical phase, that is, an optical signal modulated in DQPSK, can be outputted, as denoted at B1 and B2 in FIG. 31.
Like the RZ pulse-curving intensity modulator denoted by a reference character 114 in FIG. 30, the RZ pulse-curving intensity modulator 136 transforms the optical signal obtained by combining the modulated beams from the phase modulators 135-1 and 135-2 into an RZ pulse. Particularly, an optical signal transformed into an RZ pulse by use of a clock driving signal which is at the same frequency (21.5 GHz) as the bit rate and has an amplitude one times as large as the distinction voltage (Vπ) is called an RZ-DQPSK signal, whereas an optical signal transformed into an RZ pulse by use of a clock driving signal which is at a frequency (10.75 GHz) one half of the bit rate and has an amplitude two times as large as the distinction voltage (Vπ) is called a CSRZ-DQPSK signal.
The optical receiving apparatus 140 is connected to the optical transmitting apparatus 130 over a transmission path 101 to perform received signal processing on a (CS)RZ-DQPSK signal from the optical transmitting apparatus 130. The optical receiving apparatus 140 comprises a branching unit 147 for branching the received optical signal, together with delay interferometers 141-1 and 141-2, photoelectric converting units 142-1 and 142-2, and regenerating circuits 143-1 and 143-2 along the optical signal paths branched by the branching unit 146. The optical receiving apparatus 140 further comprises a multiplexing unit (2:1 MUX) 144 for multiplexing data signals regenerated by the regenerating circuits 143-1 and 143-2, and a received data processing unit 145.
Signals obtained by branching the (CS)RZ-DQPSK signal transmitted over the transmission path 101 are inputted to the delay interferometers 141-1 and 141-2. The delay interferometer 141-1 makes a component delayed by one bit time (46.5 ps in this case) and a component whose phase is controlled to be at π/4 radian interfere with each other, and outputs two results of the interference. The delay interferometer 141-2 makes a components delayed by one bit time and a component whose phase is controlled to be at −π/4 radian (shifted by π/2 from the phase in the delay interferometer 141-1) interfere with each other, and outputs two results of the interference.
Each of the delay interferometers 141-1 and 141-2 may be composed of a Mach-Zehnder interferometer. One of the branching waveguides of each of the Mach-Zehnder interferometers is formed to have a length longer than the other branching waveguide by a propagation length corresponding to one bit time, and each of the delay interferometers 141-1 and 141-2 has an electrode 141a or 141b for controlling the phase of an optical signal propagated in the latter branching waveguide.
The photoelectric converting unit 142-1 is composed of a dual pin photodiode for performing balanced detection by receiving the two outputs from the delay interferometer 141-1. Similarly, the photoelectric converting unit 142-2 is composed of a dual pin photodiode for performing balanced detection by receiving the two outputs from the delay interferometer 141-2. Received signals detected by the above photoelectric converting units 142-1 and 142-2 are appropriately amplified by amplifiers 142e. 
The regenerating circuit 143-1 regenerates I (In-phase) components of a clock signal and a data signal from the optical signal received by the photoelectric converting unit 142-1. The regenerating circuit 143-2 regenerates Q (Quadrature-phase) components of a clock signal and a data signal from the optical signal received by the photoelectric converting unit 142-2.
The multiplexing unit 144 is inputted the IQ components of the clock signals and the data signals from the regenerating circuits 143-1 and 143-2 to convert them into a 43 Gbit/s data signal before modulated in DQPSK. On the basis of the data signal from the multiplexing unit 144, signal processing such as error correction and the like is performed in the received data processing unit 145.
In the above (CS)RZ-D(Q)PSK modulation/demodulation system, in order to convert a phase modulated signal into an intensity modulated signal in the optical receiving apparatus 120 or 140, the delay interferometer 121, or 141-1 and 141-2 gives a delay difference of one bit time to the signal to cause optical interference. To obtain a desired optical signal in the delay interferometer 121, or 141-1 and 141-2 at this time, it is necessary to appropriately set the phase of an optical signal to be interfered with a component delayed by one bit time.
For example, RZ-DPSK or CSRZ-DPSK requires that the phase of an optical signal to be interfered with a component delayed by one bit time in the delay interferometer 121 shown in FIG. 30 be set to 0 radian, whereas (CS)RZ-DQPSK requires that the phases of optical signals to be interfered with components delayed by one bit time in the delay interferometers 141-1 and 141-2 be set to π/4 and −π/4 radians, respectively.
Other techniques relating to the present invention are described in the following patent documents 1 through 5:
(Patent Document 1) U.S. Patent Application Publication 2004-0223749;
(Patent Document 2) Japanese Unexamined Patent Application Publication No. HEI 8-321805;
(Patent Document 3) Japanese Unexamined Patent Application Publication No. 2000-115077;
(Patent Document 4) Japanese Unexamined Patent Application Publication No. 2003-60580; and
(Patent Document 5) Published Japanese Translation of PCT International Publication for Patent Application No. 2004-516743
In 40 Gb/s or 43 Gb/s transmission, the above optical receiving apparatus is required to have a severe wavelength dispersion tolerance such as about 1/16 of that at the time of 10 Gb/s transmission. For this, it is necessary to provide a variable chromatic dispersion compensator (VDC) 150 at the receiving end as shown in FIGS. 30 and 31 to perform highly accurate dispersion compensation.
In such case, it is necessary to optimally set both the phase control in the delay interferometer and a dispersion compensation amount in the variable dispersion compensator. Namely, when an optical signal undergone CS(RZ)-D(Q)PSK is received, it is necessary to optimally set both the delay interferometer and the variable dispersion compensator in order to demodulate the modulated optical signal.
For dispersion compensation, it is assumed that the number of errors is monitored, using the number of error corrections relating to the received signal decoded, and the variable dispersion compensator is controlled on the basis of the number of monitored errors.
However, the characteristics of the dispersion compensation amount and the characteristics of the phase control amount with respect to the number of errors are different in nature, and the control amounts in both the delay interferometer and the variable dispersion compensator are shifted from the optimum values in the stage of initial setting. For this, there is a problem that a long time is required to search optimum control amounts for both the delay interferometer and the variable dispersion compensator to obtain good quality of the received signal, which is harm to quickly stabilize the control amounts in the delay interferometer and the variable dispersion compensator.
Namely, since the number of errors above mentioned varies according to the optical phase control by the delay interferometer and the control on the dispersion compensation amount by the variable dispersion compensator, it is difficult to quickly stabilize the control amounts in the both after the initial start of the apparatus.
Further, since the transmission path chromatic dispersion and the optical phase difference in the delay interferometer vary with temperature fluctuation and the like during the system operation, it is necessary to adaptively control the delay interferometer and the variable dispersion compensator. The techniques described in the above patent documents 1 through 5 and other known techniques do not examine the control by both the delay interferometer and the variable dispersion compensator in the phase modulation system.